In x-ray projection imaging, attenuation is a function of photon energy. The extent of this phenomenon differs among materials. For example, although iodine and calcium exhibit similar x-ray attenuations at high energy levels (e.g., 150 keV), iodine demonstrates a larger increase in x-ray attenuation vis-a-vis calcium at lower energy levels.
Computed Tomography (CT) may exploit this difference via multiple image volumes (i.e., “energy images”) acquired based on different ranges of photon energies to facilitate discrimination between the materials. There are several ways to acquire these “energy images.” In general, either the spectra emitted from the x-ray tube is changed, as in Dual Energy CT (better described as Dual kV CT), or, as in Photon Counting CT, there is only one incident spectrum but the detector(s) thresholds the photons into different energy categories after they have passed thru the material (i.e., a patient).
The specific method of acquiring the energy images, the energy levels themselves, and the number of different energy images acquired determine the efficiency and/or accuracy of material discrimination. Suitable values differ depending on the available energies, particular materials to be discriminated, and system noise. Systems are desired which provide efficient determination of imaging photon energy level combinations for discriminating two or more given materials.